The LPR pressure should be set to around 85psi - never more than 100psi.
If the LPR pressure is constant, just too high, then there probably isn't anything wrong with it - it just needs turning down. If it starts low and then works up, then it could be an issue with the valve core, or could mean that one of the o-rings on the bolt is leaking, and allowing HP and LP air to mix, causing the LPR pressure to increase.
Assuming that the tank output pressure isn't creeping, then it is the HPR that sounds more like it has a valve core issue. If the valve core is leaking, that would prevent the reg from regulating correctly, and allow higher pressures to build up inside the marker, which would explain the increase in velocity.
If you have another inline reg, try it on the Quest, and see if it resolves the problem.
Another possibility is that the HPR piston is sticking and holding the valve open longer than it should - clean the reg, grease the piston, and make sure that the piston o-ring isn't swollen. Having said that, I would expect the piston to stick more to begin with and to loosen up as you fire, not the reverse as seems to be happening here, so I'm more inclined to think that it is an issue with the valve.